Light emitting device

ABSTRACT

To enhance the emission output of the light emitting device including an active layer made of nitride semiconductor containing In, the light emitting device having an active layer between the n-type semiconductor layer and the p-type semiconductor layer, characterized in that the active layer comprises an well layer made of In x1 Ga 1−x1 N (x 1 &gt;0) containing In and a first barrier layer made of Al y2 Ga 1−y2 N (y 2 &gt;0) containing Al formed on the well layer.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a light emitting device provided with anitride semiconductor (for example, In_(x)Al_(y)Ga_(1−x−y)N, 0≦x, 0≦y,x+y≦1) including light emitting devices such as LED (light emittingdiode) and LD (laser diode).

[0003] 2. Prior Art

[0004] Recently, the light emitting device including nitridesemiconductor layers such as a blue LED or LD has attracted attention.Since the device made of nitride semiconductor has a high meltingtemperature and a relatively high heat resistance, it has a smalltemperature dependence and is expected to find application in not onlylight emitting devices but also various kinds of devices.

[0005] Also, the LED has excellent properties such as low powerconsumption and long lifetime and has an effect on the economy of powerconsumption and the decrease of maintenance frequency. Therefore, theLED holds promise as a light emitting source for the traffic signals anda high bright LED which can give sufficient visual identificationoutdoors is expected to be developed. The light in the yellow region is,often used in an indicator that attracts attention such as directionalsignals of the car and a bulletin board for traffic information otherthan the aforementioned traffic signal.

[0006] The LED made of AlGaInP has been already realized as the highbright LED which emits the light having a wavelength in the yellowregion. However, the LED made of AlGaInP had large temperaturedependence and particularly, the emission output thereof decreasedextremely at an elevated temperature. Such a decrease of the emissionoutput at an elevated temperature becomes a large problem, particularlyin the indicator which is mounted outdoors. This is because generally,the temperature within the indicator is very high in summer when thesolar radiation is very strong or in the regions such as tropic zonesand the decrease of the emission output in the situations that the solarradiation is very strong leads to the decrease of the visualidentification. Such a fact becomes an extremely large problem for theindicator which emits light in the yellow region and is often used as anindicator that attracts attention.

[0007] It is known that the active layer is made of mixed crystalcontaining indium (In) and the mixing ratio x of In is large to decreasethe band gap energy, with the result that the wavelength of the emittedlight is long, so as to emit the light having a wavelength in the yellowregion corresponding to yellow of the traffic signals in the lightemitting device including nitride semiconductor layers (for example,In_(x)Al_(y)Ga_(1−x−y)N, 0≦x, 0≦y, x+y≦1) However, if the wavelength ofthe emitted light is made long by increasing the mixing ratio of indiumin the active layer to emit the light having a wavelength in the yellowregion using the light emitting device having nitride semiconductorlayers, the problem of decrease the emission output with increase of themixing ratio of indium comes up. The emission output decreases markedlyfrom the point where the emitting wavelength λd is about 550 nm.Further, the active layer including indium (In) in the amount requiredto achieve an emitting wavelength of about 590 nm has an extremely badsurface state and then, the problem that the semiconductor layers havinga good crystallinity cannot be formed on such a surface thereof comesup.

SUMMARY OF THE INVENTION

[0008] This invention has been accomplished to solve the above-mentionedproblems. It is an object of the present invention to enhance theemission output of the light emitting device including an active layermade of nitride semiconductor containing In, particularly, of the lightemitting device which emits a light having a wavelength longer (notless, than 550 nm) than that in the yellow region. It is another objectof the present invention to enhance the crystallinity of thesemiconductor layers which are formed on the nitride semiconductor layerwhich emits the light having a wavelength in the yellow region.

[0009] To achieve the above-mentioned object, according to the presentinvention, there is provided a light emitting device having an activelayer between the n-type semiconductor layer and the p-typesemiconductor layer, characterized in that the active layer comprises anwell layer made of In_(x1)Ga_(1−x1)N (x1>0) containing In and a firstbarrier layer made of Al_(y2)Ga_(1−y2)N (y2>0) containing Al formed onthe well layer.

[0010] In the light emitting light device having such a structureaccording to the present invention, the emission output of the nitridesemiconductor light emitting device, particularly of the light emittingdevice comprising a nitride semiconductor layer which emits a lighthaving a wavelength longer than that in the yellow region can beenhanced. Also, the semiconductor layers formed on the well layer andthe first barrier layer can be improved in crystallinity.

[0011] Also, in the light emitting device according to the presentinvention, the well layer is made of ternary mixed crystal,In_(x1)Ga_(1−x1)N (0.6≦x1≦1), resulting in that the well layer and thefirst barrier layer formed thereon can be improved in crystallinity andthe semiconductor layers formed on the first barrier layer can befurther improved in crystallinity.

[0012] In the light emitting device according to the present invention,the well layer may contain Al that is diffused from the adjacent layer.

[0013] In the light emitting device according to the present invention,the first barrier layer may contain In that is diffused from theadjacent layer.

[0014] In the light emitting device according to the present invention,the mixing proportion x1 of In in the well layer is controlled to be notless than 0.6, resulting in the light emission of a wavelength in theyellow region or longer.

[0015] Also, in the light emitting device according to the presentinvention, the mixing proportion x1 of In in the well layer ispreferably adjusted to obtain the light emission having a wavelength ofnot less than 530 nm.

[0016] Further, in the light emitting device according to the presentinvention, the mixing proportion y of Al in the first barrier layer ispreferably not less than 0.1, resulting in, particularly, theimprovement of the emission output of the device which emits the lighthaving a wavelength in the yellow region or longer.

[0017] And the mixing proportion y of Al in the first barrier layer ismore preferably adjusted to be not less than 0.15, most preferably notless than 0.2.

[0018] Thus, the increase of the mixing proportion y2 of Al in the firstbarrier layer enables the threshold voltage to be decreased.

[0019] And in the light emitting device according to the presentinvention, the active layer preferably comprises a second barrier layermade of In_(x3)Al_(y3)Ga_(1−x3−y3)N, (0≦x3≦0.3, 0≦y3≦0.1, x3+y3≦0.3) andthereby, the well layer formed thereon can be improved in crystallinity.

[0020] And in the light emitting device according to the presentinvention, the second barrier layer is more preferably made of ternarymixed crystal, In_(x3)Ga_(1−x3)N (0≦x3≦0.3) or binary mixed crystal, GaNwhich corresponds to x3=0 and thereby, the crystal defects due to thedifference in the lattice constant between the barrier layer and thewell layer can be decreased and the deterioration of the crystallinityof the second barrier layer itself due to high mixed crystallization ofIn can be prevented, resulting in the improvement of the well layer incystallinity.

[0021] Since the light emitting device according to the presentinvention comprises the second barrier layer, the well layer and thefirst barrier layer, even when the mixing proportion of In is large, theactive layer having a good crystallinity can be formed. Such an activelayer may be more suitable for the active layer in themulti-quantum-well structure.

[0022] And in the light emitting device according to the presentinvention, it is preferable that the n-type semiconductor layercomprises an n-type cladding layer to confine the carrier within theactive layer and the p-type semiconductor layer comprises a p-typecladding layer to confine the carrier within the active layer, thedevice comprising an n-side second cladding layer made of nitridesemiconductor containing In between the active layer and the n-typecladding layer and a p-side second cladding layer made of nitridesemiconductor containing In between the active layer and the p-typecladding layer.

[0023] In the light emitting device in such a configuration, the n-sidesecond cladding layer and the p-side second cladding layer can preventthe deterioration of the crystallinity and the occurrence of theundesirable distortion in the active layer due to the difference in thelattice constant between the n-type cladding layer and the p-typecladding layer, resulting in the enhancement of the emission output.

[0024] As described above, according to the present invention, theemission output of the nitride semiconductor device, particularlythe,nitride semiconductor light emitting device which emits the lighthaving a wavelength longer than that in the yellow region (not less than550 nm) can be enhanced. The cystallinity of the nitride semiconductorlayers on the nitride semiconductor layer from which the light having awavelength in the yellow region is emitted can be improved.

[0025] Particularly, in the case that the mixing proportion of Al, y2 inthe first barrier layer is not less than 0.1, that is, y2≧0.1, theabove-mentioned effect is brought to the fore. Further, the effect isalso produced on the decrease of the threshold voltage of the lightemitting device on condition that y2≧0.15, preferably y2≧0.2.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIG. 1 is a schematic cross sectional view of the light emittingdiode of the first embodiment according to the present invention,

[0027]FIG. 2 is a schematic cross sectional view showing theconfiguration of the active layer of the first embodiment,

[0028]FIG. 3 is a schematic cross sectional view showing theconfiguration of the active layer of a modified example of the firstembodiment,

[0029]FIG. 4 is a schematic cross sectional view showing theconfiguration of the active layer of another modified example of thefirst embodiment,

[0030]FIG. 5 is a schematic cross sectional view showing theconfiguration of the active layer of the second embodiment,

[0031]FIG. 6 is a schematic cross sectional view showing theconfiguration of the active layer of a modified example of the secondembodiment,

[0032]FIG. 7 is a schematic cross sectional view showing theconfiguration the active layer of another modified example of the secondembodiment,

[0033]FIG. 8 is a schematic cross sectional view showing theconfiguration of the laser diode of the third embodiment according tothe present invention,

[0034]FIG. 9 is a graph showing the relation between the driving voltageand the mixing proportion, y2 of Al of the first barrier layer, in thelight emitting diode according to the present invention,

[0035]FIG. 10 is a graph showing the relation between the emissionoutput and the mixing proportion, y2 of Al of the first barrier layer,in the light emitting diode according to the present invention,

[0036]FIG. 11 is a graph showing relation between the PL emission outputand the mixing proportion, y2 of Al of the first barrier layer, in thelight emitting diode according to the present invention, in theintermediate state of the device,

[0037]FIG. 12 is a graph showing the temperature characteristics of thelight emitting diode according to the present invention, compared tothose of the conventional light emitting diode made of AlGaIn,

[0038]FIG. 13 is a schematic cross sectional view of the configurationof the light emitting diode of the fourth embodiment according to thepresent invention,

[0039]FIG. 14 is a AFM photograph showing the surface morphology of thefirst barrier layer (in which mixing proportion of Al is 0.15) 13 of thelight emitting diode according to the present invention,

[0040]FIG. 15 is a AFM photograph showing the surface morphology of thefirst barrier layer (in which mixing proportion of Al is 0.30) 13 of thelight emitting diode according to the present invention,

[0041]FIG. 16 is a AFM photograph showing the surface morphology of thefirst barrier layer (in which mixing proportion of Al is 0.45) 13 of thelight emitting diode according to the present invention,

[0042]FIG. 17 is a AFM photograph showing the surface morphology of thefirst barrier layer (in which mixing proportion of Al is 0.60) 13 of thelight emitting diode according to the present invention,

[0043]FIG. 18 is a graph showing the driving voltage versus the mixingproportion Z of Al in the light emitting diode according to the presentinvention,

[0044]FIG. 19 is a schematic cross sectional view showing theconfiguration of the active layer of the fourth embodiment according tothe present invention,

[0045]FIG. 20 shows the energy level of the active layer and the regionsclose to the active layer of the fourth embodiment according to thepresent invention,

[0046]FIG. 21 is a schematic cross sectional view showing theconfiguration of the active layer of the fifth embodiment according tothe present invention,

[0047]FIG. 22 is a schematic cross sectional view showing theconfiguration of the active layer of a modified example of the fifthembodiment according to the present invention,

[0048]FIG. 23 is a graph showing the relation between the emissionoutput and the wavelength in the light emitting diode of the fifthembodiment according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0049] Embodiment 1

[0050]FIG. 1 is a schematic view showing the configuration of the lightemitting diode according to the first embodiment of the presentinvention. The light emitting diode has such a configuration as a bufferlayer 102, an undoped GaN layer 103, an n-type contact layer 104, ann-type cladding layer 105, an active layer 106, a p-type cladding layer107 and a p-type contact layer 108 are successively laminated on asubstrate 101 and an n-electrode and a p-electrode are formed on then-type contact layer and the p-type contact 108, respectively.

[0051] The substrate 101 is used to form desirable nitride semiconductorlayers (In_(x)Al_(y)Ga_(1−x−y)N, 0≦x, 0≦y, x+y≦1) thereon and selectedappropriately. The buffer layer 102 is formed for the purpose ofrelaxing lattice constant mismatch between the substrate 101 and thenitride semiconductor (In_(x)Al_(y)Ga_(1−x−y)N, 0≦x, 0≦y, x+y≦1). Theundoped GaN layer 103 is formed for the purpose of improving the n-typecontact layer 104 which is to be formed thereon in crystallinity. Then-type contact layer 104 is formed for the purpose of realizing theohmic contact. The n-type cladding layer 105 is formed for the purposeof confining the carrier within the active layer 106. The active layer106 is formed to emit light. The p-type cladding layer 107 is formed forthe purpose of confining the carrier within the active layer 106, likethe n-type cladding layer 105. The p-type contact layer 108 is formedfor the purpose of realizing the ohmic contact with the p-electrode 112.

[0052] The substrate 101, the buffer layer 102 and the undoped GaN layer103 of the above-mentioned light emitting device may be removedselectively, if necessary, for the purpose of the enhancement of theemission output and the decrease of internal absorption of the light,after the formation of each layer on said substrate and said layers. Inthis specification, The growing direction of each layer is defined asupper direction. Therefore, in the embodiment 1, the p-typesemiconductor layers are in the upper direction from the n-typesemiconductor layers.

[0053] Components of the nitride semiconductor light emitting diode ofthe first embodiment according to the present invention will now bedescribed in details below.

[0054] The substrate 101 may be made of, in addition to sapphire havingthe principal plane in C plane, sapphire having principal plane in Rplane or A plane, insulating substrate such as spinel (MgAl₂O₄), orother semiconductor substrate such as SiC(including 6H, 4H 3C), ZnS,ZnO, GaAs and GaN.

[0055] The buffer layer 102 is formed for the purpose of relaxinglattice constant mismatch between the substrate 101 and the nitridesemiconductor (In_(x)Al_(y)Ga_(1−x−y)N, 0≦x, 0≦y, x+y≦1) and forexample, is formed by growing AlN, GaN, AlGaN, InGaN, etc. The bufferlayer 102 may be formed at a temperature of not more than 900° C. to athickness of 10 angstroms to 500 angstroms.

[0056] The undoped GaN layer 103 is made as a GaN layer which is notdoped with an impurity. According to the Embodiment 1, the undoped PaNlayer 103 is further formed on the buffer layer 102, to improve incrystallinity the n-type contact layer 104 which is to be formed on theundoped GaN layer 103.

[0057] The n-type contact layer 104 is usually doped with an n-typeimpurity such as Si in a concentration of not less than 3×10¹⁸/cm³ andpreferably not less than 5×10¹⁸/cm³. The composition of the n-typecontact layer 104 may be In_(x5)Al_(y5)Ga_(1−x5−y5)N, (0≦x5, 0≦y5,x5+y5≦1) , but the present invention is not limited to such acomposition. The n-type contact layer may be preferably made of GaN orAl_(y)Ga_(1−y)N (the value of y is not more than 0.2) to obtain anitride semiconductor layer with few crystal defects. The thickness ofthe n-type contact layer 104 is controlled to be 0.1 to 20 μm,preferably 0.5 to 10 μm, and more preferably 1 to 5 μm so as to form ann-electrode 111 thereon, but is not limited to the above-mentionedrange.

[0058] Next, the n-type cladding layer 105 is preferably in thestructure comprising at least three layers of a undoped n-type claddingfirst layer, a n-type cladding second layer doped with an n-typeimpurity and a undoped n-type cladding third layer. And the n-typecladding layer 105 may further comprise layers other than theaforementioned first to third layers. The n-type cladding layer 105 maybe close to the active layer 106. Otherwise, there may be providedanother layer between the n-type cladding layer and the active layer106.

[0059] The n-type cladding first to third layers may be made of variouscomposition of nitride semiconductor (In_(x)Al_(y)Ga_(1−x−y)N, 0≦x, 0≦y,x+y≦1), but may be preferably made of GaN. The composition of one of thefirst to third layers of the n-type cladding layer 104 may be differentfrom that of the other layers or may be the same as that of the otherlayers. The thickness of the n-type cladding layer 105 is controlled tobe preferably 175 to 12000 angstroms, more preferably 1000 to 10000angstroms and most preferably 2000 to 6000 angstroms, but is not limitedto the above-mentioned range. If the thickness of the n-type claddinglayer 105 is within the above-mentioned range, the optimization of Vfcan be achieved and the withstand static voltage can be enhanced.

[0060] The total thickness of the cladding layer 105 may be preferablycontrolled to be within the aforementioned range by controlling eachthickness of the first to third layers of the n-type cladding layerappropriately. Each layer which constitutes the n-type cladding layer105 may be made of semiconductor having a composition ofIn_(x)Al_(y)Ga_(1−x−y)N, (0≦x, 0≦y, x+y≦1), preferably a compositionIn_(x)Al_(y)Ga_(1−x−y)N having a low mixing proportion of In and Al,more preferably GaN.

[0061] In the first embodiment, the active layer 106 having a singlequantum well structure, as shown in FIG. 2, is formed on the n-typecladding layer 105. The active layer 106 comprises a second barrierlayer 11 which is formed under the well layer 12, an well layer 12 wherethe carrier is recombined and the light emitted and a first barrierlayer 13 which is formed on the well layer 12. Further, the uppermostlayer 14 is formed on the first barrier layer 13 to grow the p-typecladding layer 107 having a good crystallinity.

[0062] The second barrier layer 11 is made of, for example, nitridesemiconductor represented by the general formula,In_(x3)Al_(y3)Ga_(1−x3−y3)N, (0≦x3, 0≦y3, x3+y3≦0.1) including 2 or moreelements selected from indium (In), aluminum (Al), gallium (Ga) andnitrogen (N). The second barrier layer 11 preferably has a mixingproportion x3 or y3 of In or Al, respectively, of x3≦0.3, y3≦0.3,x3+y3≦0.3, in order to form thereon an well layer 12 which has a goodcrystallinity and is made of nitride semiconductor having a large mixingproportion of In required to emit light having a wavelength in theyellow wavelength. In particular, the second barrier layer is preferablymade of ternary mixed crystal of In_(x3)Ga_(1−x33)N, (x3≦0.3) or binarymixed crystal of GaN to decrease the crystal defects due to thedifference in lattice constant between the second barrier layer 11 andthe well layer 12 and to prevent the deterioration in crystallinity ofthe lower barrier layer 11 itself as the mixing proportion of Inincreases. In such a case, the cystallinity of the well layer 12 whichis formed on the second barrier layer 11 can be enhanced. The thicknessof the second barrier layer 11 is preferably 10 angstroms to 100angstroms to improve the well layer 12 in crystallinity.

[0063] The well layer 12 is made of, for example,In_(x1)Al_(y1)Ga_(1−x1−y1)N, (x1>0, y1≦0, x1+y1≦1) including indium(In), aluminum (Al), gallium (Ga) and nitrogen (N). The mixingproportion of indium (In), aluminum (Al) and gallium (Ga) is determinedin such a manner that the peak wavelength, λd is within the range offrom 550 nm to 610 nm or the range beyond such a range, where theeffective yellow light emission can be obtained. In particular, themixing proportion, x1 of indium (In) in the well layer 12 is controlledto be x1≧0.6 to obtain a preferable wavelength of yellow light.

[0064] However, as the mixing proportion x1 of In increases, the latticeconstant increases and the lattice mismatch between the well layer andthe other layers increases. Therefore, the well layer 12 is needed to beformed on the layer having a relatively small lattice constantdifference from that of the well layer 12 and having a goodcrystallinity. According to the present invention, the well layer 12 isformed in combination with the above-mentioned second barrier layer 11and therefore, the well layer 12 having a good crystallinity can beformed. Moreover, even the layer of which the lattice constant islargely different from that of the well layer 12 can be formed to have agood crystallinity. The well layer 12 may be preferably made of ternarymixed crystal of In_(x1)Ga_(1−x1)N or binary mixed crystal of GaN, so asto grow the layer having a good crystallinity. The thickness of the welllayer 12 is preferably 10 angstroms to 100 angstroms.

[0065] The first barrier layer 13 is made of, for example,In_(x2)Al_(y2)Ga_(1−x2−y2)N, (x2≧0, y2≧0, x2+y2≦1) including indium(In), aluminum (Al), gallium (Ga) and nitrogen (N). The first barrierlayer 13 includes aluminum (Al) as an essential element and therefore,the output of the light having a wavelength in the yellow region whichis emitted in the well layer 12 can be enhanced. When the first barrierlayer 13 is formed of such a composition, the uppermost layer 14 havinga good crystallinity can be formed on the first barrier layer 13. Thefirst barrier layer 13 may be preferably made of ternary mixed crystalof Al_(y2)Ga_(1−y2)N, so as to grow the layer having a goodcrystallinity. The mixing proportion of Al is preferably y2≧0.1, morepreferably y2≧0.15. The band gap energy of the first barrier layer 13 ispreferably higher than that of the second barrier layer 11. Thethickness of the first barrier layer 13 is preferably 10 angstroms to100 angstroms.

[0066] The uppermost layer 14 is made of, for example,In_(x4)Al_(y4)Ga_(1−x4−y4)N, (0≦x4, 0≦y4, x4+y4≦0.1) including indium(In), aluminum (Al), gallium (Ga) and nitrogen (N). The uppermost layer14 is preferably controlled to have a mixing proportion of In or Al of0≦x4, y4≦0.1 and x4+y4≦0.1, so as to grow a p-type cladding layer havinga good crystallininty on the uppermost layer 14. Moreover, the uppermost layer 14 may be preferably made of ternary mixed crystal ofAl_(y4)Ga_(1−y4)N or binary mixed crystal of GaN, so as to grow thep-type cladding layer 107 having a good crystallinity thereon. Thethickness is preferably 10 angstroms to 100 angstroms. The uppermostlayer 14 may have the same constitution as that of the second barrierlayer 11.

[0067] The above-mentioned each layer of the active layer 106 may beselected appropriately from an undoped layer without an impurity, ann-type doped layer which is doped with an n-type impurity such as Si anda p-type doped layer which is doped with a p-type impurity such as Mg.

[0068] According to the first embodiment, the active layer is made bylaminating successively the second barrier layer 12, the well layer 12,the first barrier layer 13 and the uppermost layer 14 on the n-typecladding layer 105. However, the present invention is not limited tothis configuration. And instead of the active layer, as shown in FIG. 3,the active layer 106 a can be used which is made by laminatingsuccessively the first barrier layer 13, the second barrier layer 11,the well layer 12, the first barrier layer 13, the uppermost layer 14.Thus, the first barrier layer 13 having a band gap energy higher thanthat of the second barrier layer 11 is formed on the n-type claddinglayer 105 side with respect to the well layer 12 to confine the carriermore effectively within the well layer 12.

[0069] As long as the lattice constant of the first barrier layer 13matches the lattice constant of the p-type cladding layer 107, insteadof the active layer, as shown in FIG. 4, the active layer 106 b in whichthe uppermost layer 14 is omitted can be used. Certainly, the activelayer may be formed by laminating the first barrier layer 13, the secondbarrier layer 11, the well layer 12 and the first barrier layer.

[0070] In other words, according to the present invention, there may beprovided only a first barrier layer 13 containing Al on at least onewell layer. As long as the minimum requirements can be satisfied,various modifications can be made.

[0071] A p-type cladding layer 107 is formed on the active layer 106.The p-type cladding layer 107 may be a multi-layered film form bylaminating the the p-type cladding first layer having a higher band gapenergy and the p-type cladding second layer having a band gap energylower than that of the first layer, or may be a single layer made ofAl_(b)Ga_(1−b)N, (0≦b≦1) containing a p-type impurity. When amulti-layered film, the concentration of the p-type impurity of thep-type cladding first layer may be different from or the same as that ofthe p-type cladding second layer.

[0072] Next, the p-type cladding layer 107 having a multi-layeredstructure (super lattice structure) will be described in details below.

[0073] The thickness of the p-type cladding first layer and the p-typecladding second layer which constitute the p-side cladding layer 107which is a multi-layered film is controlled to be not more than 100angstroms, preferably not more than 70 angstroms and more preferablywithin the range of 10 to 40 angstroms. The thickness of the p-typecladding first layer may be the same as or different from that of thep-type cladding second layer. When the thickness of each layerconstituting the multi-layered film is controlled to be within theabove-mentioned range, the thickness of each layer is within the elasticstrain limit and the nitride semiconductor layer having a bettercrystallinity can be grown compared to the case of a large thickness.Since the nitride semiconductor layer having a good crystallinity can begrown, when a p-type impurity is doped, the p-type layer having a largecarrier concentration and a small resistivity can be obtained, resultingin the decrease of Vf and threshold of the device. Two kinds of layershaving such a thickness is defined as one pair and the multi-layeredfilm is formed by laminating the pairs several times. The totalthickness of the p-type multi-layered cladding layer is controlled byadjusting the thickness of each layer of the p-type cladding first layerand the p-type cladding second layer and adjusting the lamination times.The total thickness of the p-type multilayered cladding layer is notspecified, but is not more than 2000 angstroms, preferably not more than1000 angstroms, and more preferably not more than 500 angstroms.

[0074] The p-type cladding first layer is desirably made by growing thenitride semiconductor containing at least Al, preferably Al_(n)Ga_(1−n)N(0<n≦1) and the p-type cladding second layer is desirably made bygrowing the nitride semiconductor of binary mixed crystal or ternarymixed crystal such as AlpGa1−pN (0≦p<1,n>p) and In_(r)Ga_(1−r)N (0≦r≦1).

[0075] In the p-type cladding layer, the p-type impurity concentrationof the p-type cladding first layer is different from that of the p-typecladding second layer. One layer has a larger impurity concentration andthe other layer had a smaller impurity concentration. As in the case ofthe n-type cladding layer 105, the p-type cladding first layer having ahigher band gap energy preferably has a p-type impurity concentrationlarger than that of the p-type cladding second layer having a lower bandgap energy. The p-type cladding second layer having a lower band gapenergy is preferably an undoped layer. However, the present invention isnot limited to the above-mentioned configuration. And the p-typecladding first layer having a higher band gap energy may have a p-typeimpurity concentration smaller than that of the p-type cladding secondlayer having a lower band gap energy.

[0076] The amount of doping in the p-type cladding first layer ispreferably controlled within a range from 1×10¹⁸/cm³ to 1×10²¹/cm³, ormore preferably within a range from 1×10¹⁹/cm³ to 5×10²⁰/cm³. When theimpurity concentration is lower than 1×10¹⁸/cm³, the carrierconcentration is low. On the other hand, when the impurity concentrationis higher than 1×10²¹/cm³, the crystallinity deteriorates. Meanwhile thep-type impurity concentration in the p-type cladding second layer may beat any level as long as it is lower than that of the p-type claddingfirst layer, but it is preferably lower than on tenth of the latter.Most preferably the p-type cladding second layer is undoped, in whichcase a layer of high mobility can be obtained. However, because thep-type cladding second layer is thin, some of the p-type impuritydiffuses from the p-type cladding first layer into the p-type claddingfirst layer. Therefore, the p-type impurity concentration in the p-typecladding second layer including the above-mentioned diffusing impurityis desirably within 1×10²⁰/cm³ and lower than that in the p-typecladding first layer. The effect is the same also in case that thep-type cladding first layer having a higher band gap energy is dopedwith less amount of p-type impurity and the p-type cladding second layerhaving a lower band gap energy is doped with greater amount of p-typeimpurity.

[0077] The p-type impurity is selected from among the elements of IIAgroup and IIB group of the periodic table such as Mg, Zn, Ca and Be andpreferably selected from among Mg, Ca and the like.

[0078] In component nitride semiconductor layer constituting the p-typecladding layer 107 in the multi-layered structure, the layer doped withthe impurity in a higher concentration is preferably doped so that sucha distribution of impurity concentration is obtained, that the impurityconcentration is high in the middle portion of the semiconductor layerin the direction of thickness and is low (preferably undoped) in theportions near the ends, resulting in the,decrease of the resistivity.

[0079] Next, the p-type cladding layer 107 in the single layer structuremade of Al_(b)Ga_(1−b)N (0≦b≦1) containing a p-type impurity will bedescribed bellow. The thickness of the p-side cladding layer 107 iscontrolled to be within 2000 angstroms, preferably within 1000 angstromsand more preferably within a range from 100 to 500 angstroms.

[0080] The p-type cladding layer 107 in the single layer structure has alittle worse crystallinity compared to the p-type cladding layer in themulti-layered structure. The p-type cladding layer 107 in the singlelayer structure in combination with the n-type cladding layer 105 in themulti-layered structure can provide a good crystallinity. Thus, the useof such a single layer in combination with other layer configuration canprevent the deterioration of the device properties. Such a single layercan allow the manufacturing process to be simplified and is suitable tomass production.

[0081] The p-type impurity concentration in the p-type cladding layer107 in the single layer structure is controlled within a range from1×10¹⁸/cm³ to 1×10²¹/cm³, preferably within a range from 5×10¹⁸/cm³ to5×10²⁰/cm³, and more preferably within a range from 5×10¹⁹/cm³ to1×10²⁰/cm³. When the impurity concentration is within the aforementionedrange, a good p-type film can be obtained.

[0082] According to the present invention, it is preferable that thep-type contact layer 108 is doped with Mg and the composition of thelayer is binary mixed crystal nitride semiconductor, GaN, including noindium and no aluminum. If the p-type contact layer includes In or Al,good ohmic contact thereof with the p-electrode 112 can be obtained,resulting in the decrease of luminous efficiency. The thickness of thep-side contact layer 108 is within the range from 0.001 μm to 0.5 μm,preferably within the range from 0.01 μm to 0.3 μm and more preferablywithin the range from 0.05 μm to 0.2 μm. When the thickness of thep-side contact layer is less than 0.001 μm, a short circuit may easilyoccur between the p-electrode 112 and the p-type GaAlN cladding layer,resulting in the deterioration of function of the p-side contact layer108. Since the GaN contact layer of binary mixed crystal which has acomposition different from that of the contact layer is formed on theGaAlN cladding layer of ternary mixed crystal, in the case of thethickness being more than 0.5 μm, crystal defects tend to occur in thep-side GaN contact layer 9 due to crystal mismatch, resulting in thedeterioration of crystallinity. In term of the luminous efficiency, theless the thickness of the contact layer, the lower Vf is, with theresult that the luminous efficiency can be enhanced. The p-type impuritywhich is doped in the p-type GaN contact layer 9 is preferably Mg, andthereby, the p-type characteristic can be easily achieved and the ohmiccontact can be easily achieved. The concentration of Mg is controlledwithin a range of from 1×10¹⁸/cm³ to 1×10²¹/cm³, preferably within arange from 5×10¹⁹/cm³ to 3×10²⁰/cm³, and more preferably to about1×10²⁰/cm³.

[0083] The p-type contact layer 108 may be composed of a p-type contactfirst layer made of undoped GaN and a p-type contact second layer madeof Mg doped GaN.

[0084] The n-electrode 111 is formed on the n-type contact layer 104,and the p-electrode 112 is formed on the Mg doped p-type GaN contactlayer 9. In the present invention, the material of the n- andp-electrodes 111, 112 is not specified but for example, the n-electrode111 may be made of W/Al and the p-electrode 112 may be made of Ni/Au.

[0085] In the light emitting diode of the first embodiment, because theactive layer 16 comprises a first barrier layer on an well layer, thecarrier can be more efficiently confined within the well layer toenhance the luminous efficiency.

[0086] Embodiment 2

[0087] The light emitting diode of the second embodiment according tothe present invention will be described below.

[0088] As described above, since the well layer 12 and the layer formedon the first barrier layer can have a good crystallinity, the presentinvention can be applied to the multi quantum well structure.

[0089] The light emitting device of the second embodiment according tothe present invention is a light emitting diode comprising an activelayer 106 c having a multi quantum well structure, as shown in FIG. 5,instead of the active layer 106 of the light emitting device of thefirst embodiment. The light emitting diode of the second embodiment hasa similar configuration to that of the first embodiment except for theactive layer.

[0090] In the light emitting device of the second embodiment, the activelayer 106 c having a multi quantum well structure is formed bylaminating a second barrier layer 11, an well layer 12 and a firstbarrier layer in this order over and over again. In this case, anuppermost layer 14 is formed after the top first barrier layer isformed. The second barrier layer 11, the well layer 12, the firstbarrier layer 13 and the uppermost layer 14 which constitute the activelayer 106 c are formed in the same way as in the first embodiment.

[0091] The light emitting device of the second embodiment which has sucha configuration has excellent properties as described below.

[0092] In the prior yellow light emitting device without the firstbarrier layer containing Al, the active layer was in the form of a multiquantum well structure including several well layers and thecrystallinity is deteriorated, resulting in the decrease of the emissionoutput.

[0093] However, in the yellow light emitting device comprising a firstbarrier layer containing Al as an essential element, even if the activelayer is formed in a multi quantum well structure including several welllayers 12, the emission output can be enhanced, in contrast to the priordevice.

[0094] The configuration in which the second barrier layer 11, the welllayer 12 and the first barrier layer 13 are formed in this order overand over again on the n-type cladding layer 105 and finally, theuppermost layer 14 is formed has been described. However, according tothe present invention, as shown in FIG. 6, the first barrier layer 13 isformed on the n-type cladding layer 105, and then, the active layer 106d may be made by laminating the second barrier layer 11, the well layer12 ant the first barrier layer in this order over and over again andfinally forming the uppermost layer 14 like in FIG. 5. Thus, when thefirst barrier layer 13 having a band gap energy higher than that of thesecond barrier layer 11 in the side of the n-type cladding layer withrespect to the first well layer 12, the carrier can be confined moreeffectively within the first well layer 12.

[0095] As long as the lattice constant of the first barrier layer 13matches that of the p-type cladding layer 107, the uppermost layer 14may be omitted in the active layer 106 d of FIG. 6, as shown in FIG. 7.The active layer having such a configuration is designated as 106 e inFIG. 7. Certainly, in the active layer 106 d which is formed bylaminating the second barrier layer 11, the well layer 12 and the firstbarrier layer in this order over and over again after the format-on ofthe first barrier layer 13 on the n-type cladding layer 105, theuppermost layer 14 may also be omitted.

[0096] As described above, in the light emitting device according to thepresent invention, various modifications can be made as long as a firstbarrier layer containing Al is formed on at least one well layer.

[0097] Therefore, the present invention can be applied to the laserdevice as described below.

[0098] Embodiment 3

[0099] The light emitting laser device of the third embodiment accordingto the present invention will be described.

[0100]FIG. 8 is a schematic view showing the configuration of thesemiconductor laser device according to the third embodiment of thepresent invention.

[0101] In the nitride semiconductor laser device of the thirdembodiment, a buffer layer 202, an n-type contact layer 203, an n-typecladding layer 204, an n-type optical waveguide layer 205, a p-typeoptical cladding layer 208 and a p-type contact layer 209 are formedsuccessively on the substrate 201 and an n-electrode 211 and ap-electrode 212 are formed on the n-type contact layer 204 and thep-type contact layer 208, respectively.

[0102] The substrate 201, the buffer layer 202, the n-type contact layer203 and the n-type cladding layer 204 are formed in the same manner asin the first and second embodiments. In the third embodiment, an undopedGaN layer may formed between the buffer layer 202 and the n-type contactlayer 203, in a like way as in the first and the second embodiment.

[0103] The n-type optical waveguide layer 205 constitutes an opticalwaveguide in conjunction with the active layer 206. Therefore, then-type optical waveguide layer is formed in such a manner that therefractive index of the n-type optical waveguide layer 205 is notlargely different from that of the active layer 206, but is different toa sufficient degree from that of the n-type cladding layer 204.

[0104] In the third embodiment, the active layer 206 is formed in thesame way as in the first embodiment. However, the present invention isnot limited to this configuration and the active layer 206 may be formedin a multi quantum well structure as in the case of the secondembodiment. The multi quantum well layer can allow the output toincrease.

[0105] The p-type optical waveguide layer 205 constitutes an opticalwaveguide in conjunction with the active layer 206 in a like manner asthe n-type optical waveguide layer 105. Therefore, the p-type opticalwaveguide layer 207 is formed in such a manner that the refractive indexof the n-type optical waveguide layer 207 is not largely different fromthat of the active layer 206, but is different to a sufficient degreefrom that of the p-type cladding layer 208.

[0106] The p-type cladding layer 208 and the p-type contact layer 209are formed in the same manner as in the first and second embodiments. Inthe case of the laser, the p-type cladding layer 208 and the p-typecontact layer 209 are etched to the vicinity of the boundary surface ofthe p-type optical waveguide layer 207 to form a ridge geometry with astripe width of 1.5 μm (the stripe ridge), with the result that theemission from the active layer 206 focuses under the stripe ridge andthe thresholds can be decreased. Particularly, the layers above thep-type cladding layer are preferably in the form of the ridge stripe.

[0107] The n-electrode 211 and the p-electrode 212 are formed in thesame manner as in the first and second embodiments. Further, aninsulting film 230 is formed on the outer surface of the laser.

[0108] The nitride semiconductor laser device of the third embodimentwhich has such a configuration as described above comprises an firstbarrier layer containing Al on the well layer, as in the case of thefirst and second embodiments, resulting in the decrease of thethresholds.

[0109] Embodiment 4

[0110] The light emitting device (light emitting diode) of the fourthembodiment according to the present invention has the same configurationas that of the light emitting diode of the first embodiment, as shown inFIG. 13, except that the active layer 106 f which will be describedbellow in details is provided instead of the active layer and an n-typemulti-layered film 105 a is further provided between the active layer106 f and the n-type cladding layer 105.

[0111] In FIG. 13, elements similar to those of the first embodiment aredesignates as like number.

[0112] In the light emitting device of the fourth embodiment, the activelayer 106 f is constituted, as shown in FIG. 19, by laminating thesecond barrier layer 11, the well layer 12 and the first barrier layer13 in this order several times on the n-type multi-layered film 105 aand finally, forming the second barrier layer 11 on the topmost layer.

[0113] The fourth embodiment is particularly characterized in that thefirst barrier layer 13 is made of Al_(z)Ga_(1−z)N having a mixingproportion of Al of not less than 0.30 to enhance the effect the firstbarrier layer 13.

[0114] In other words, we found that the higher effect of the firstbarrier layer 13 is brought to the fore by growing the first barrierlayer 13 made of Al_(z)Ga_(1−z)N (0.30≦Z≦1) and then raising thetemperature to the growing temperature of the second barrier layer 11and applied the discovery to the LED device of the fourth embodiment.

[0115] We also found that there is a relation between the effect of thefirst barrier layer 13 and the surface morphology thereof.

[0116] FIGS. 14 to 17 show the photographs of the surface morphology ofthe first barrier layer 13 which is obtained by forming the firstbarrier layer 13 at 820° C. and then, raising the temperature to 1050°C., observed by AFM (atomic force microscope) . FIG. 14 shows thephotograph of the case that the mixing proportion Z of Al is 0.15, FIG.15 shows Z=0.30, FIG. 16 shows Z=0.45, and FIG. 16 shows Z=0.60. Asshown in figures, when the mixing proportion of Al is not less than 0.30(Z≧0.30), the surface of the first barrier layer 13 is in the meshstructure having several regions which cave in or are bored through thelayer. The reasons for this are as follows. Since the first barrierlayer made of AlGaN is formed at a low temperature, the crystallinityand the thickness of the AlGaN layer is uneven. When the temperature israised to the temperature for growing the second barrier layer after theformation of the first barrier layer 13, In of the well layer 12 isdecomposed in the region of AlGaN which has a bad crystallinity and asmall thickness. Thereby, the surface of the first barrier layer 13caves in or is bored through the layer. Thus, a part of the surface ofthe well layer 12 is exposed. It was also found that when the mixingproportion Z of Al is not less than 0.30, the regions which cave in orare bored through the layer occupy not less than 10% of the surface ofthe first barrier layer 13 and the driving voltage is extremelydecreased. The results are shown in FIG. 18. FIG. 18 is a graph whichshows the change of the driving voltage with the variation of the mixingratio of Al in the nitride semiconductor LED device according to thepresent invention.

[0117] The preferable thickness of the component layer of the activelayer in the fourth embodiment is like to that in the first embodiment.

[0118] In the fourth embodiment, the active layer 106 f in a multiquantum well structure will be described, but the same effect can beobtained in the case of the active layer 106 f is in a single quantumwell structure.

[0119] In the active layer 106 f of the fourth embodiment 4, the bandgap energy of the first barrier layer 13 is controlled, as shown in FIG.20, to be higher than that of the second barrier layer 11 and the firstbarrier layer 13 is surely formed close to the well layer 12 after theformation of the well layer 12.

[0120] In the fourth embodiment, the first barrier layer 13 is formed onevery well layer 12 as a most preferable example. But the presentinvention is not limited to this configuration and it is essential onlythat a first barrier layer 13 is formed at least one among the placesbetween the well layer 12 and the second barrier layer 11.

[0121] The thickness of the first barrier layer 13 is smaller than thatof the second barrier layer 11 and is preferably a monoatomic layer orlonger and not more than 100 angstroms. If the thickness is beyond 100angstroms, a mini-band is formed between the first barrier layer 13 andthe well layer 12, resulting in the deterioration of the luminousefficiency. Therefore, the thickness of the first barrier layer 13 isdesirably as small as possible.

[0122] The thickness of the second barrier layer 11 can be controlledwithin the range from 10 angstroms to 400 angstroms. The thickness ofthe well layer 12 is preferably controlled within the range from 10angstroms to 70 angstroms.

[0123] In the fourth embodiment, the second cladding layer 105 a, amulti-layered film, is, for example, an undoped layer in the superlattice structure constructed of the undoped GaN layer and the undqpedInGaN layer, which is formed for the purpose of enhancing the emissionoutput.

[0124] The n-type cladding layer 105 is formed in the same manner as inthe first embodiment to increase the withstand static voltage.

[0125] Thus, in the fourth embodiment, the n-side cladding layer isconstructed of the n-type cladding layer 105 and the second claddinglayer 105 a, a multi-layered film to increase the emission output andthe withstand static voltage.

[0126] As described above, in the light emitting diode of the fourthembodiment, there is provided a first barrier layer containing Al in therelatively large amount which has a band gap energy higher than that ofthe second barrier layer to decrease the driving voltage. And the n-sidecladding layer is constructed of the n-type cladding layer 105 and thesecond cladding layer 105 a to increase the emission output and thewithstand static voltage.

[0127] The forth embodiment has been described with reference to thenitride semiconductor LED device, but the present invention is notlimited to the LED device. Even when the present invention is applied tothe nitride semiconductor laser device, the similar effect can beachieved.

[0128] Embodiment 5

[0129] Next, the light emitting diode of the fifth embodiment accordingto the present invention will be described.

[0130] The light emitting diode of the fifth embodiment is fabricated inthe same manner as the light emitting diode of the second embodiment,except that an n-side second cladding layer 21 made of nitridesemiconductor containing In is formed between the n-type cladding layer105 and the active layer 106 c, and a p-side second cladding layer 22made of nitride semiconductor containing In is formed between the p-typecladding layer 105 and the active layer 106 c.

[0131] In the fifth embodiment, the n-side second cladding layer 21 andthe p-side second cladding layer 22 are formed on each side of theactive layer 106 c to prevent the deterioration of the crystallinity andthe occurrence of the undesirable strains in the active layer 106 cresulting from the lattice constant mismatch between the n-type claddinglayer 105 and the p-type cladding layer 107.

[0132] Therefore, the n-side second cladding layer 21 and the p-sidesecond cladding layer 22 are effective particularly in the case that thelattice constant of the n-type cladding layer 105 is largely differentfrom that of the p-type cladding layer 107 and the active layercontaining In in the relatively large amount is formed.

[0133] To be more specific, the n-side second cladding layer 21functions in such a manner that the well layer 12 is formed with Inbeing dispersed evenly, with the result that the variation of thecomposition of the well layer can be decreased. This configuration canprevent the decrease of the emission output of the light emitting devicewhich comprises an active layer including an well layer of highly mixedcrystal which contains In in a relatively large amount. This effect isbrought to the fore, particularly in the case that the active layer 106c in a multi quantum well structure in which the well layer 12 ofrelatively highly mixed crystal containing relatively high proportion ofIn must be formed repeatedly is grown.

[0134] The n-side second cladding layer 21 may be made of nitridesemiconductor containing In represented by the general formulaIn_(X6)Ga_(1−X6)N, The above-mentioned effect can be exerted better bysetting the X6 within a range from 0.0025 to 0.1 and setting thethickness within a range from 1000 Å to 5000 Å.

[0135] The p-side second cladding layer 22 acts to relax the undesirablestrain in the well layer of the active layer which occurs at the timewhen the p-type cladding layer 107 is formed on the active layer 106 cand which results from the lattice constant mismatch between the activelayer 106 c and the p-type cladding layer 107. This action can,particularly, prevent the decrease of the emission output of the lightemitting device which comprises an active layer including an well layerof highly mixed crystal containing In in a relatively large amount. Thiseffect is brought to the fore, particularly in the case that the activelayer 106 c in a multi quantum well structure in which the well layer 12of relatively highly mixed crystal containing relatively high proportionof In must be formed repeatedly is grown.

[0136] The p-side second cladding layer 21 may be made of nitridesemiconductor containing In represented by the general formulaIn_(X7)Ga_(1−X7)N. The above-mentioned effect can be exerted better bysetting the X7 within a range from 0.005 to 0.1 and setting thethickness within a range from 100 Å to 1000 Å.

[0137]FIG. 23 is a graph showing the emission output Po (mW) versus thewavelength λd (nm) of the light emitting diode of the fifth embodimentaccording to the present invention, compared to the conventional lightemitting diode.

[0138] As shown in the graph of FIG. 23, the configuration of the fifthembodiment can prevent the decrease of the emission output in thewavelength range of not less than 535 nm.

[0139] An assessment was made of the light emitting diode of the presentinvention and the conventional light emitting diode which hadbelow-mentioned configurations, respectively.

[0140] The light emitting diode of the present invention has such aconfiguration that comprises:

[0141] (1) buffer layer 102; undoped Al_(x)Ga_(1−x)N, 100 angstroms,

[0142] (2) undoped GaN layer 103; GaN, 1.5 μm,

[0143] (3) n-type contact layer 104; Si doped GaN, 4.165 μm

[0144] (4) n-type cladding layer 105; three layered structureconstructed of undoped GaN (3000 angstroms)/Si doped GaN (300angstroms)/undoped GaN (50 angstroms),

[0145] (5) n-side second cladding layer ; In_(0.005)G_(0.995)N, 3500angstroms,

[0146] (6) active layer 106 c; (a second barrier layer made of GaN, 70angstroms/an well layer made of InGaN, 30 angstroms/a first barrierlayer made of AlGaN, 40 angstroms)×4 cycle+an uppermost layer made ofGaN, 70 angstroms,

[0147] (7) p-side second cladding layer 22; Mg dopedIn_(0.014)Ga_(0.986)N, 200 angstroms,

[0148] (8) p-type cladding layer 107; undoped Al_(0.042)Ga_(0.958)N,2500 angstroms, and

[0149] (9) p-type contact layer 108; Mg doped GaN, 1200 angstroms.

[0150] The conventional light emitting diode has such a configurationthat comprises:

[0151] (1) buffer layer; undoped Al_(x)Ga_(1−x)N, 100 angstroms,

[0152] (2) undoped GaN layer; GaN, 1.5 μm,

[0153] (3) n-type contact layer; Si doped GaN, 4.165 μm

[0154] (4) n-type cladding layer; three layered structure constructed ofundoped GaN (3000 angstroms)/Si doped GaN (300 angstroms)/undoped GaN(50 angstroms),

[0155] (5) buffer super lattice layer; (GaN, 40angstrom/In_(0.13)Ga_(0.87)N, 20 angstroms)×10+GaN, 40 angstroms,

[0156] (6) active layer 106 c; (a barrier layer made of GaN, 200angstroms/an well layer made of InGaN, 30 angstroms)×4 cycle+a barrierlayer made of GaN, 200 angstroms,

[0157] (7) p-side cladding layer; Mg doped Al_(0.16)Ga_(0.84)N, 40angstroms/Mg doped In_(0.03)Ga_(0.97)N, 25 angstroms)×5+Mg doped Al_(0.16)Ga_(0.84)N, 40 angstroms,

[0158] (8) undoped AlGaN; undoped A1 _(0.05)Ga_(0.95)N, 2800 angstroms,and

[0159] (9) p-type contact layer; Mg doped GaN, 1200 angstroms.

[0160] In the light emitting diode of the fifth embodiment, the activelayer 106 c is formed in such a way that a second barrier layer is firstformed on the n-side second cladding layer 21. But, according to thepresent invention, as shown in FIG. 22, the active layer 106 c may beformed in such a way that a first barrier layer is first formed on then-side second cladding layer 21.

[0161] The same effect can be achieved in the above-mentionedconfiguration, like in the fifth embodiment.

EXAMPLES

[0162] The examples of the present invention will be described below.

[0163] To make sure, the present invention is not limited to theexamples.

Example 1

[0164] The device of Example 1 is fabricated in the following manner.The C-plane sapphire substrate is used as a substrate 101. The componentlayer is grown using a metal-organic chemical vapor deposition (MOCVD)method. Trimethylgallium (TMG), triethylgallium (TEG), trimethylindium(TMI) and trimethylaluminum (TMA) is used as a source of III-groupelement, Ga, In and Al, respectively. Ammonia (NH₃)is used as a sourceof V-group element, N. Monosilane (SiH₄) is used as a source of ann-type dopant and bicyclopentadienyl magnesium (Cp₂Mg) as a source of ap-type dopant, respectively. H₂ and N₂ are used as a carrier gas and aflowing gas.

[0165] First, an auxiliary substrate 101 made of sapphire is set in theMOCVD device and the temperature of the substrate 101 is increased to1140° C. in H₂ to treat the surface of the substrate 101.

[0166] After the surface treatment of the substrate, the temperature ofthe substrate 101 is decreased to 510° C. A buffer layer 102 made of GaNhaving a thickness of about 200 angstroms is grown on the substrate 101using TMG (trimethylgallium) and ammonia as a source of GaN.

[0167] After growing the buffer layer 102, the temperature of thesubstrate is increased to 1150° C. Using TMG and ammonia as a source ofGaN, an undoped GaN layer 103 is grown to the thickness of 1.5 μm on thebuffer layer.

[0168] Further, after forming an undoped GaN layer 103, Using TMG andammonia as a source of GaN and SiH₄ as a source of Si of the dopant, ann-type contact layer 104 made of GaN doped with Si to 5×10¹⁸/cm³ isgrown to the thickness of about 2 μm.

[0169] Next, an n-type cladding layer 105 is formed on the n-typecontact layer 104. The n-type cladding layer 105 is constructed of an-type cladding fist layer, a second layer and a third layer. An undopedGaN layer is formed as an n-type cladding first layer and a third layerusing TMG and NH₃ as a source of GaN. A GaN layer doped with Si to5×10¹⁸/cm³ is formed as an n-type cladding second layer using SiH₄ as asource of Si of the dopant. The thickness of the n-type cladding firstlayer, second layer and third layer is 3000 angstroms, 300 angstroms and50 angstroms, respectively.

[0170] Next, the temperature of the substrate is decreased to 1000° C.,and TEG in the amount of 40 cc/min and NH₃ in the amount of about 3l/min are supplied instead of TMG. Thus, a second barrier layer 11 madeof undoped GaN having a thickness of 50 angstroms is formed on then-type contact layer 104. In the case that the second barrier layer ismade of ternary mixed crystal of In_(X3)Ga_(1−X3)N, TMA in anappropriate amount is further supplied.

[0171] Next, the temperature is decreased to 750° C. TEG in the amountof 4.5 cc/min, NH₃ in the amount of about 3 l/min and TMI in the amountof 40 cc/min are supplied. Thus, an well layer 12 made ofIn_(0.75)Ga_(0.25)N having a thickness of 35 angstroms is formed on thesecond barrier layer 11.

[0172] And then, the temperature of the substrate is increased to 800°C. and a first barrier layer 13 is formed. TEG in the amount of 18cc/min, NH₃ in the amount of about 3 l/min and TMA in the amount of 4.5cc/min are supplied. Thus, a first barrier layer 13 made ofAl_(0.2)Ga_(0.8)N having a thickness of 30 angstroms is formed on thewell layer 12.

[0173] The process for growing the above-mentioned first barrier layer11, well layer 12 and first barrier layer 13 are repeated four times.And finally, the temperature of the substrate is increased to 1000° C.and TEG in the amount of 40 cc/min and NH₃ in the amount of about 3l/min are supplied. Thereby, an uppermost layer 14 made of undoped GaNhaving a thickness of about 50 angstroms is formed on the topmost firstbarrier layer 13. Thus, the active layer 106 in the multi quantum wellstructure is formed.

[0174] Next, after forming the uppermost layer 14, at the sametemperature, a p-type cladding layer 197 made of Mg dopedAl_(0.1)Ga_(0.9)N having a thickness of about 200 angstroms is formed onthe active layer 106 using TEG, NH₃ and TMA as a reactive gas and CP₂Mgas a dopant source.

[0175] Next, the temperature is decreased to 950° C. and a p-typecontact first layer 197 made of undoped GaN having a thickness of about1300 angstroms is formed on the cladding layer using TEG and NH₃ as areactive gas.

[0176] Further, CP₂Mg is added as a dopant source to form a p-typecontact second layer made of Mg doped GaN having a thickness of about200 angstroms on the p-type contact first layer.

[0177] Then, the inside of the reactor is replaced with N₂ and at 600°C., the heat annealing is conducted for 5 minutes. The heat annealingallows the p-type cladding layer 107 and p-type contact layer 108 to betransformed into the p-type layer having a high carrier concentration.

[0178] Next, the etching is conducted until the n-type contact layer 104is exposed, in order to form the n-electrode 111. An n-electrode 111 anda p-electrode 112 are formed in the predetermined place on the wafer,respectively.

[0179] The yellow light emitting diode as fabricated in this waycomprises an well layer 106 made of In_(0.75)Ga_(0.25)N showed theresults of emission peak wavelength λd of 590 nm, half width of 45 nm,emission output of 1.8 mW and driving voltage of 3.2 V under a forwardcurrent (If) of 20 mA.

[0180] Example 2

[0181] The light emitting diode of Example 2 is fabricated in the sameway as in Example 1 except that the active layer is formed in thefollowing manner.

[0182] According to Example 2, after forming an n-type cladding layer105, the temperature of the substrate is decreased to 1000° C. and TEGin the amount of 40 cc/min and NH₃ in the amount of about 3 l/min aresupplied instead of TMG. Thus, a second barrier layer 11 made of undopedGaN having a thickness of 50 angstroms is formed on the n-type contactlayer 104.

[0183] Next, the temperature of the substrate is decreased to 750° C.and TEG in the amount of 4.5 cc/min, NH₃ in the amount of about 3 l/minand further, TMI in the amount of 40 cc/min are supplied. Thus, an welllayer 12 made of In_(0.75)Ga_(0.25)N having a thickness of about 35angstroms is formed on the second barrier layer 11.

[0184] And the temperature of the substrate is increased to 800° C. toform a first barrier layer 13. TEG in the amount of 18 cc/min, NH₃, inthe amount of about 3 l/min and further, TMA in the amount of 4.5 cc/minare supplied. Thus, a first barrier layer 13 made of Al_(0.2)Ga_(0.8)Nhaving a thickness of about 30 angstroms is formed on the well layer 12.

[0185] Thereafter, the temperature of the substrate is increased to1000° C. and TEG in the amount of 40 cc/min and NH₃ in the amount ofabout 3 l/min are supplied. Thereby, an uppermost layer 14 made ofundoped GaN having a thickness of about 50 angstroms is formed on thefirst barrier layer 13. Thus, the active layer 106 in a single wellquantum structure is formed.

[0186] After forming the uppermost layer 14, the p-type cladding layerand the layers above the p-type cladding layer 107 are formedsequentially in the same manner as in Example 1.

[0187] The light emitting diode of Example 2 fabricated in theabove-mentioned way showed the similar results to those in Example 1.

[0188] Next, the light emitting diode of Example 1 having a multiquantum well structure will described in which the first barrier layeris made of Al_(y2)Ga_(1−y2)N with a mixing proportion of Al being variedsuccessively. FIG. 9 shows the relationship between the driving voltageand the mixing proportion, y2. The driving voltage as described hereinmeans a driving voltage required to drive the light emitting diode underthe forward current If=20 mA. As shown in the drawing, the voltagerequired to drive the light emitting device under the forward current of20 mA tends to decrease in the region of y2<0.15 and becomes almostconstant in the region of y2≧0.15. This indicates that the effect of theincrease of the mixing proportion of Al in the first barrier layer onthe decrease of the driving voltage reaches the saturation point in theregion of y2≧0.15.

[0189] Next, FIG. 10 shows the emission output versus the mixingproportion y2 of Al of the light emitting diode of Example 1 having amulti quantum well structure in which the first barrier layer is made ofAl_(y2)Ga_(1−y2)N. As shown in the drawing, the emission output isenhanced extremely in the region of y2≧0.1.

[0190] Generally, the difference ΔE of the band gap energy (=Eg1−Eg2)required to confine the carrier in the laser diode having adouble-hetero structure made AlGaAs and InGaAsP is ΔE≧0.3 eV. (HirooYonezu, optical communication device engineering—light emitting deviceand light receiving device, pp.72, 1.8-1.14, Kougakutosho co.publication.

[0191] The Japanese Patent Laid-Open Publication No. Hei6-164055discloses that the semiconductor layer comprising an well layer and abarrier layer made of In_(x)Al_(y)Ga_(1−x−y)N (0≦x, 0≦y, x+y≦1)preferably the difference ΔE of the band gap energy between them of notless than 0.3 eV.

[0192] The difference of the band gap energy ΔE of the light emittingdiode of the present invention made of In_(x)Al_(y)Ga_(1−x−y)N isestimated roughly using the following approximation expression:

Eg=3.4−1.45x|2.8y(eV)

[0193] In the case that the second barrier layer 11 is made of GaN andthe well layer is made of In_(0.75)Ga_(0.25)N, ΔE is about 1.1 eV.Further, the well layer 12 is made of In_(0.75)Ga_(0.25)N and the firstbarrier layer 13 is made of Al_(0.2)Ga_(0.8)N, ΔE is about 1.6 eV.

[0194] The carrier confinement in the laser diode needs to be conductedat higher density than that in the light emitting diode. ΔE required forthe laser diode is higher than that for the light emitting diode.Considering the above-mentioned facts, the second and first barrierlayers 11, 13 in the light emitting diode according to the presentinvention has an extremely large band gap energy difference from thewell layer 12, compared to the general barrier layer.

[0195] ΔEg between the first barrier layer 13 and the well layer 12 isminimal in the case that the mixing proportion y2 of Al in the firstbarrier layer 13 and the mixing proportion x1 of In in the well layer 12are small. Even when y2 is 0.1, the lower limit for having a largeeffect on the enhancement of emission output and x1 is 0.6, the lowerlimit for obtaining a preferable yellow emission wavelength, ΔEg isabout 1.2 eV, which is still extremely large. Further, even when y2 is0.15, the lower limit for having an effect on the decrease of thethreshold voltage and x1 is 0.6, the lower limit for obtaining apreferable yellow emission wavelength, ΔEg is about 1.3 eV, which is alarger value.

[0196] When the first barrier layer having a large mixing proportion, y2of Al is formed on the well layer 12 having a large mixing proportion,x1 of In, the emission output of the light emitting diode can beenhanced extremely. The reason for this is considered to lie in theexceedingly large difference in the band gap energy between the welllayer and the barrier layer. Then, we conducted PL evaluation using adevice in the intermediate state with no p-type semiconductor layer,which had an active layer 106 composed of one second barrier layer 11,one well layer 12 and one first barrier layer, in order to assess thedevice eliminating various other factors. As shown in FIG. 11, the PLemission output versus the mixing proportion of Al in the first barrierlayer was enhanced in the region of y2 being not less than about 0.12.It is supposed from this fact that the effect of enhancing the emissionoutput of the light emitting diode comprising nitride semiconductorlayers which emits the light having a wavelength in the yellow region isachieved mainly by forming the first barrier layer having a large mixingproportion, y2 of Al on the well layer. In other words, theabove-mentioned effect is ascribable to an exceedingly large differencein the band gap energy between the well layer and the barrier layer inthe active layer.

[0197] Further, in the aforementioned Japanese Patent Laid-OpenPublication No. Hei6-164055, it is described that the difference inlattice constant between the well layer and the barrier layer of thesemiconductor laser which comprises the well layer and the barrier layermade of In_(x)Al_(y)Ga_(1−x−y)N (0≦x, 0≦y, x+y≦1) is preferably not morethan 1%. On the other hand, the well layer made of In_(0.6)Ga_(0.4)N andthe first barrier layer made of Al_(0.1)Ga_(0.9)N have a latticeconstant of 3.402 angstroms and 3.181 angstroms, respectively and theratio of the difference in lattice constant between both layers to thelattice constant of the well layer is about 6.5%. The ratio is muchlarger than the difference in lattice constant of 1%, which is apreferable value in the Japanese Patent Laid-Open Publication No.Hei6-164055. The well layer made of In_(0.6)Ga_(0.4)N and the firstbarrier layer made of Al_(0.2)Ga_(0.8)N have a lattice constant of 3.402angstroms and 3.174 angstroms, respectively and the ratio of thedifference in lattice constant is about, 6.7%, which is further larger.However, the inventors found that the second barrier layer of thepresent invention can improve the crystallinity of the well layer whichis formed on the second barrier layer and the first barrier layer havinga lattice constant which is largely different from that of the welllayer and has a good crystallinity can be formed on the well layer.Further, the second barrier layer is formed one more time on the firstbarrier layer, with the result that a multi quantum well structure canbe formed without the deterioration of the crystallinity. From anotherviewpoint, the present invention may be based on the effect of thelattice mismatch resulting from the large difference in lattice constantbetween the well layer and the first barrier layer.

[0198] Next, FIG. 12 shows the comparison in the temperaturecharacteristics between the light emitting diode of the presentinvention which comprises nitride semiconductor layers and emits lightin the yellow region and the conventional light emitting diode made ofAlGaInP. The light emitting diode comprising nitride semiconductorlayers in FIG. 12 is an example of the light emitting diode of thisexample, which has an active layer 106 in a multi quantum well structurecomprising a second barrier layer 11 made of undoped GaN, an well layermade of In_(0.75)Ga_(0.25)N and a first barrier layer 13 made ofAl_(0.2)Ga_(0.8)N. The temperature characteristics in FIG. 12 isrepresented as an emission output under the driving current If=20 mA. Asshown in the drawing, the deterioration of the emission output at anelevated temperature in the light emitting diode of the presentinvention made of nitride semiconductor is smaller than that in theconventional light emitting diode made of AlGaInP.

[0199] When the indicator such as a traffic signal constructed of lightemitting diodes is used outdoors, the light emitting diode need to havea large emission output, in term of an visual identification, in thesituation of the solar radiation being very strong. Generally, since thetemperature within the indicator is very high in summer when the solarradiation is very strong or in the regions such as tropic zones, theindicator which is used outdoors is preferably constructed of lightemitting devices of which the decrease of the emission output is smallat an elevated temperature. When the indicator is used outdoors, thetemperature inside the indicator often reaches 75° C. The emissionoutput at 75° C. of the light emitting diode made of AlGaInP decreasesby about 50%, compared to at room temperature, 25° C. The emissionoutput at 75° C. of the nitride semiconductor light emitting diodeaccording to the present invention maintains about not less than 80%,compared to at 25° C. Thus, the present invention can prevent thedecrease of the emission output.

[0200] Thus, the nitride semiconductor light emitting device accordingto the present invention which emits yellow light has more excellenttemperature characteristics, compared to the light emitting diode madeof AlGaInP which emits yellow light.

Example 3

[0201]FIG. 13 is a schematic sectional view showing the structure of thenitride semiconductor LED device according to one example of the presentinvention. Example 3 will be described with reference to FIG. 3. Thestructure of the light emitting device according to the presentinvention in not limited to that of FIG. 13.

[0202] (Buffer Layer 202)

[0203] The GaN substrate 101 (which may be a sapphire substrate) whichhas been formed using a widely known method on the substrate made ofC-face sapphire of two-inch φ is set in the MOVPE reactor. A bufferlayer 102 made of GaN which has a thickness of about 200 angstroms isgrown using TMG and ammonia on the GaN substrate 101.

[0204] (Undoped GaN Layer 103)

[0205] After growing the buffer layer, TMG is stopped and thetemperature is increased to 1050° C. At to 1050° C., using ammonia andTMG as a source of GaN, an undoped GaN layer 103 is grown to thethickness of 1 μm.

[0206] (n-type Contact Layer 104)

[0207] Subsequently, at 1050° C., using TMG and ammonia as a source ofGaN and silane gas as an impurity gas, an n-type contact layer 104 madeof GaN doped with Si to 3×10¹⁹/cm³ is grown to the thickness of 4 μm.

[0208] (n-type Cladding Layer 105)

[0209] Next, only silane gas is stopped. At 1050° C., a first layer madeof undoped GaN is grown to the thickness of 3000 angstroms using TMG andammonia. Subsequently, at the same temperature, the silane gas is added.A second layer made of GaN doped with Si to 4.5×10¹⁸/cm³ is grown to thethickness of 300 angstroms. Further, subsequently, only silane gas isstopped and at the same temperature, a third layer made of undoped GaNis grown to the thickness of 50 angstroms. Thus, an n-type claddinglayer 105 in the three-layered structure having a total thickness of3350 angstroms is formed.

[0210] (n-type Multi-layered Film 105 a)

[0211] (n-side Optical Waveguide Layer 8)

[0212] Next, at the same temperature, a second nitride semiconductorlayer made of undoped GaN is grown to the thickness of 40 angstroms. Andthen, the temperature is decreased to 800° C. and a first nitridesemiconductor layer made of undoped In_(0.13)Ga_(0.87)N is grown to thethickness of 20 angstroms using TpG, TMI and ammonia. These operationsare repeated and the first and second layers are laminated alternatelyin the order of the second layer+the first layer, in 10 layers each. Andfinally, the second nitride semiconductor layer made of GaN is grown tothe thickness of 40 angstroms. Thus, the n-type multi-layered film 105 ain a super lattice structure having a total thickness of 640 angstromsis formed.

[0213] (Active Layer 106 f)

[0214] Next, at 1050° C., a second barrier layer 11 made ofIn_(0.1)Ga_(0.9)N doped with Si to 5×1¹⁷/cm³ is grown to the thicknessof 200 angstroms using TMG, TMI, ammonia and silane gas. Subsequently,at 820° C., a well layer 12 made of In_(0.3)Ga_(0.7)N is grown to thethickness of 30 angstroms using TMG, TMI and ammonia. Further, a firstbarrier layer 13 made of undoped Al_(0.3)Ga_(0.7)N is grown to thethickness of 10 angstroms using TMG, TMA and ammonia. This three-layeredstructure constructed of the second barrier layer 11, the well layer 12and the first barrier layer 13 is laminated four more times and finally,the second barrier layer 11 is formed. Thus, the active layer 106 f in amulti quantum well (MQW) structure having a total thickness of 1400angstroms is formed.

[0215] (p-type Cladding Layer 107)

[0216] Subsequently, at 1050° C., a third nitride semiconductor layermade of p-type Al_(0.2)Ga_(0.8)N doped with Mg to 5×10¹⁹/cm³ is grown tothe thickness of 40 angstroms using TMG, TMA, ammonia and Cp2Mg(cyclopentadienyl magnesium). Subsequently, at 800° C., a fourth nitridesemiconductor layer made of In_(0.2)Ga_(0.98)N doped with Mg to5×10¹⁹/cm³ is grown to the thickness of 25 angstroms using TMG, TMI,ammonia and Cp2Mg. These operations are repeated and the third andfourth layers are laminated alternately in the order of the thirdlayer+the fourth layer, in 5 layers each. And finally, the third nitridesemiconductor layer is grown to the thickness of 40 angstroms. Thus, thep-type cladding layer, a multi-layered film 107 in a super latticestructure having a total thickness of 365 angstroms is formed.

[0217] (p-type Contact Layer 105)

[0218] Subsequently, at 1050° C., a p-type contact layer 108 made ofp-type GaN doped with Mg to 1×10²⁰/cm³ is grown to the thickness of 700angstroms using TMG, ammonia and Cp2Mg.

[0219] After the reaction is completed, the temperature is decreased toroom temperature. Additionally, the annealing is performed to the waferat 700° C. in nitrogen atmosphere within the reactor, so as to make thep-type layer less resistive. After annealing, the wafer is removed outof the reactor. A mask having a predetermined shape is formed on thesurface of the topmost p-type contact layer 108. The etching isconducted from the p-type contact layer side with RIE (reactive ionetching) apparatus to expose the surface of the n-type contact layer104, as shown in FIG. 13.

[0220] After etching, a translucent p-side electrode including Ni and Auand having a thickness of 200 angstroms is formed on the almost entiresurface of the topmost p-type contact layer 108, and an n-side electrode21 including W and Al on the surface of the n-type contact layer 104which has been exposed by etching, resulting in a LED device.

[0221] The resulting LED device showed a blue emission of 470 nm and adriving voltage of 3.0 V under the forward current of 20 mA. The surfacemorphology of the first barrier layer during a temperature rise up tothe growing temperature of the barrier layer is shown in FIG. 15.

Comparative Example 1

[0222] The LED device was fabricated in which the active layer wasformed in the following manner, so as to compare with Example 1.

[0223] (Active Layer)

[0224] At 1050° C., a barrier layer made of In_(0.1)Ga_(0.9)N doped withSi to 5×10¹⁷/cm³ is grown to the thickness of 200 angstroms using TMG,TMI, ammonia and silane gas. Subsequently, at 820° C., a well layer madeof In_(0.3)Ga_(0.7)N is grown to the thickness of 30 angstroms usingTMG, TMI and ammonia. Further, the barrier layer and well layer arelaminated four times and finally, the barrier layer 11 is formed. Thus,the active layer 106 in a multi quantum well (MQW) structure having atotal thickness of 1350 angstroms is formed.

[0225] Other constructions are the same as those in Example 3 exceptthat the active layer is formed in the above-mentioned manner. Theresulting device showed the broad emission peak and a driving voltage of3.8 V.

Comparative Example 2

[0226] The LED device was fabricated in which the active layer wasformed in the following manner, so as to compare with Example 1.

[0227] (Active Layer)

[0228] At 1050° C., a barrier layer made of In_(0.1)Ga_(0.9)N doped withSi to 5×10¹⁷/cm³ is grown to the thickness of 200 angstroms using TMG,TMI, ammonia and silane gas. Subsequently, at 820° C., a well layer madeof In_(0.3)Ga_(0.7)N is grown to the thickness of 30 angstroms usingTMG, TMI and ammonia. Further, a first barrier layer 13 made of undopedIn_(0.15)Ga_(0.75)N having a band gap energy which is between that ofthe carrier layer and that of the well layer is grown to the thicknessof 10 angstroms. This three-layered structure constructed of the barrierlayer, the well layer and the first barrier layer is laminated four moretimes and finally, the barrier layer is formed. Thus, the active layer106 in a multi quantum well (MQW) structure having a total thickness of1400 angstroms is formed. As mentioned above, other constructions arethe same as those in Example 3 except that the band gap energy of thefirst barrier layer 13 is lower than that of the barrier layer andhigher than that of the well layer. The resulting device showed adriving voltage of 4.0 V, which did not decrease.

Example 4

[0229] The LED device was fabricated in the same manner as in Example 3except that the first barrier layer 13 in the active layer 106 f wasmade of Al_(0.45)Ga_(0.55)N.

[0230] The resulting LED device showed a blue light emission of 470 nmunder the forward current of 20 mA and a driving voltage of 3.0 V. Thesurface morphology of the first barrier layer 13 during a temperaturerise up to the growing temperature of the barrier layer is shown in FIG.16.

Example 5

[0231] The LED device was fabricated in the same manner as in Example 3except that the first barrier layer 13 in the active layer 106 f, wasmade of Al_(0.60)Ga_(0.40)N.

[0232] The resulting LED device showed a blue light emission of 470 nmunder the forward current of 20 mA and a driving voltage of 2.8 V. Thesurface morphology of the first barrier layer 13 during a temperaturerise up to the growing temperature of the barrier layer is shown in FIG.17.

Example 6

[0233] The LED device was fabricated in the same manner as in Example 3except that the first barrier layer 13 in the active layer 106 f wasmade of Al_(0.15)Ga_(0.85)N.

[0234] The resulting LED device showed a blue light emission of 470 nmunder the forward current of 20 mA and a driving voltage of 3.6 V. Thesurface morphology of the first barrier layer 13 during a temperaturerise up to the growing temperature of the barrier layer is shown in FIG.14.

Example 7

[0235] The LED device was fabricated in the same manner as in Example 3except that the active layer 106 f was formed in the following manner.

(Active Layer 207)

[0236] At 1050° C., a second barrier layer 11 made of In_(0.1)Ga_(0.9)Ndoped with Si to 5×10¹⁷/cm³ is grown to the thickness of 200 angstromsusing TMG, TMI, ammonia and silane gas. Subsequently, at 820° C., a welllayer 12 made of In_(0.8)Ga_(0.2)N is grown to the thickness of 30angstroms using TMG, TMI and ammonia. Further, a first barrier layer 13made of undoped In_(0.3)Ga_(0.7)N is grown to the thickness of 10angstroms using TMG, TMA and ammonia. This three-layered structureconstructed of the second barrier layer 13, the well layer 12 and thefirst barrier layer 13 is laminated four more times and finally, thesecond barrier layer 13 is formed. Thus, the active layer 207 in a multiquantum well (MQW) structure having a total thickness of 1400 angstromsis formed.

[0237] As mentioned above, other constructions are the same as those inExample 3 except that the mixing proportion of In in the well layer 12is 0.8. The resulting device showed a yellow light emission of 570 nmunder a forward current of 20 mA and a driving voltage of 2.9 V, whichdecreased extremely compared to the driving voltage of 3.7 V resultedfrom the device in which no first barrier layer 13 was formed and otherconstructions were the same.

1. (Amended) A light emitting device having an n-type semiconductorlayer, a p-type semiconductor layer and an active layer between then-type semiconductor layer and the p-type semiconductor layer, whereinthe active layer comprises an well layer made of In_(x1)Ga_(1−x1)N and afirst barrier layer made of Al_(y2)Ga_(1−y2)N formed on the well layer,said x1 being set to be not less than 0.6, said y2 being set to be notless than 0.15.
 2. (Deleted)
 3. (Amended) A light emitting device havingan n-type semiconductor layer, a p-type semiconductor layer and anactive layer between the n-type semiconductor layer and the p-typesemiconductor layer, wherein the active layer comprises an well layermade of In_(x1)Ga_(1−x1)N (x1>0) and a first barrier layer made ofAl_(y2)Ga_(1−y2)N formed on the well layer, said y2 being set to be notless than 0.15, said x1 being set so that said well layer is capable ofemitting a light having a wavelength of not less than 530 nm.
 4. 5. 6.(Amended) A light emitting device according to claims 1 or 3; whereinthe y2 of said first barrier layer is set to be not less than 0.2. 7.(Amended) A light emitting device as in one of claims 1, 3, 6; whereinsaid active layer comprises a second barrier layer made ofIn_(x3)Al_(y3)Ga_(1−x3−y3)N, (0≦x3≦0.3, 0≦y3≦0.1, x3+y3≦0.3) and saidwell layer is formed on said second barrier layer.
 8. A light emittingdevice according to claim 7; wherein said second barrier layer is madeof In_(x3)Ga_(1−x3)N (0≦x3≦0.3).
 9. A light emitting device according toclaims 7 or 8; wherein said well layer, said first barrier layer andsaid second barrier layer compose a series of multi layers and whereinsaid active layer include a plurality of said series of multi layers soas to compose multiple quantum well structure.
 10. (Amended) A lightemitting device as in one of claims 1, 3, 6-9; wherein said n-typesemiconductor layer comprises an n-type cladding layer to confine thecarrier within the active layer and an n-side second cladding layer madeof nitride semiconductor containing In between said active layer andsaid n-type cladding layer, wherein said p-type semiconductor layercomprises a p-type cladding layer to confine the carrier within theactive layer and a p-side second cladding layer made of nitridesemiconductor containing In between said active layer and said p-typecladding layer.
 11. (Added) A light emitting device having an n-typesemiconductor layer, a p-type semiconductor layer and an active layerbetween the n-type semiconductor layer and the p-type semiconductorlayer, wherein the active layer comprises an well layer made ofIn_(x1)Ga_(1−x1)N (x1>0), a first barrier layer made ofAl_(y2)Ga_(1−y2)N (y2≧0.3) formed on the well layer and a second barrierlayer made of In_(x3)Al_(y3)Ga_(1−x3−y3)N, (0≦x3≦0.3, 0≦y3≦0.1,x3+y3≦0.3), said well layer being formed on said second barrier layer.12. (Added) A light emitting device according to claim 11; wherein saidsecond barrier layer is made of In_(x3)Ga_(1−x3)N (0≦x3≦0.3). 13.(Added) A light emitting device according to claims 11 or 12; whereinsaid well layer, said first barrier layer and said second barrier layercompose a series of multi layers and wherein said active layer include aplurality of said series of multi layers so as to compose multiplequantum well structure.
 14. (Added) A light emitting device as in one ofclaims 11-13; wherein said n-type semiconductor layer comprises ann-type cladding layer to confine the carrier within the active layer andan n-side second cladding layer made of nitride semiconductor containingIn between said active layer and said n-type cladding layer, whereinsaid p-type semiconductor layer comprises a p-type cladding layer toconfine the carrier within the active layer and a p-side second claddinglayer made of nitride semiconductor containing In between said activelayer and said p-type cladding layer.